Various different proximity detection techniques are usable, in theory, in the design of proximity fuses. Mention may be made of the following techniques: using the Doppler effect; using a radioaltimeter with a frequency modulated oscillator; using pulse radar; using ultrasound; using optoelectronic detection; ... .
Proximity fuses are generally constructed using a technique based on the Doppler effect. The use of this effect makes it possible to obtain a detection assembly which is relatively simple. However, use of the Doppler effect gives rise to a wide dispersion in trigger distances. This dispersion results from the fact that proximity detection using the Doppler effect relies on power amplitude discrimination in the return signal. Unfortunately, this power is related to the reflection coefficient of the obstacle under consideration and this varies very widely as a function of the nature of the obstacle. In addition, Doppler effect detection cannot be used with slow projectiles insofar as the resulting Doppler effect is unusable. In conclusion, although Doppler effect detectors are theoretically advantageous because of their simplicity, they do not give complete satisfaction because of the wide margin of uncertainty in trigger distance and because they cannot be used with slow projectiles.
Attempts have been made to remedy these drawbacks by developing radioaltimeter type detectors for proximity fuses.
The structure of one such prior art radioaltimeter type detector is shown in accompanying FIG. 1.
It can be seen in said accompanying FIG. 1 that an oscillator 1 is frequency modulated by a modulator 2. The frequency modulated oscillator 1 is coupled to an antenna 3. Thus, a frequency modulated wave is radiated towards the target. A fraction of the signal coming from the oscillator-transmitter 1 is directed to a mixer 4. The mixer also receives the echo reflected by the target. Assuming that the system fitted with the radioaltimeter is fixed relative to the target, then the echo corresponds to the transmitted wave with a delay due to the round trip time between the antenna 3 and the target. This causes the output from the mixer 4 to provide a beat frequency .DELTA.F which depends directly on the round trip time of the wave, and thus on the distance between the detector and the target. The signal obtained at the output from the mixer is applied to a discriminator circuit. The discriminator circuit may advantageously be constituted by a low frequency selector filter 5 and by a comparator 6.
By way of example, FIG. 2 has a solid line showing linear modulation of the frequency of the oscillator-transmitter 1 in the form of symmetrical sawteeth. FIG. 2 also shows a dashed line representing the received echo as directed to the mixer 4. The person skilled in the art will readily understand that a round trip time .DELTA.t corresponding to the distance d between the detector and the target gives rise to a corresponding frequency difference .DELTA.F. Thus, with linear frequency modulation of the oscillator-transmitter 1, measuring the distance d is equivalent to detecting the corresponding beat frequency .DELTA.F at the output from the mixer 4. This can be done by tuning a selective filter 5 to beats at the desired frequency .DELTA.F.
Using a radioaltimeter type detector gives rise to very much higher triggering accuracy than can be obtained with detectors that use the Doppler effect. Radioaltimeter type detectors are completely insensitive to the reflection coefficient of the target since they discriminate on the basis of a beat frequency and not on the basis of power amplitude.
The trigger distance d at which a radioaltimeter type detector responds can easily be adjusted merely by altering the slope of the low frequency signal used to frequency modulate the oscillator-transmitter.
In addition, radioaltimeter type detectors can be fitted to slow projectiles. The looked-for beat frequency .DELTA.F at the output from the mixer 4 is essentially related to the round trip time of the signal generated by the oscillator-transmitter 1, and depends, in contrast, very little on the speed of the projectile to which it is fitted. (The influence of the Doppler effect on measurements is specified below.)
In spite of all these theoretical advantages presented by radioaltimeter type detectors, it is observed, in practice, that radioaltimeter type detectors are used very little at present for making proximity fuses, whereas detectors using the Doppler effect nevertheless remain the most widely used.
This preference for Doppler effect detectors over radioaltimeter type detectors appears to be due to the fact that implementations of radioaltimeter type detectors have hitherto been bulky and very expensive since it has been necessary to select the various components of the circuit with great care, in particular the components of the oscillator-transmitter 1, in order to ensure that the modulation characteristics correspond exactly with the intended characteristics.
The object of the present invention is to provide a new radioaltimeter type detector which provides good triggering accuracy while being considerably cheaper than earlier detectors of this type.